Gold nanoclusters (Au NC) have been shown to exhibit strong antimicrobial and antibacterial properties. Nanoclusters consist of 25 gold atoms, ranging between 3-5 nm in diameter. Au nanoparticles’ (NP) enhanced properties have been used in biomedical applications such as gene therapy and diagnostics. For these properties to be efficiently utilised, their pathways into the cell must be understood. The pathways that NPs take on can be tuned by adjusting factors such as the surface functionalisation, size, composition, and shape. The intracellular pathways undertaken by these nanoparticles depend on the mechanism of cellular uptake. Most nanoparticles are expected to undergo an endocytic type of cellular uptake.
In this analysis, Au NCs capped with 6-mercaptohexanoic acid (6-MHA), were introduced to COS-7 fibroblast cells. They were then allowed to interact for 1 hour, after which a technique known as cryo-soft X-ray tomography was employed to create multiple tomograms. Using software called IMOD, 3D models of these tomograms were manually constructed. The focus was on deducing what cellular uptake mechanisms were undertaken, by identifying and comparing the general distribution patterns of Au NCs present in the samples that were modelled. From the analysed sample, 31 Au NCs were identified and their general distribution were indicative of an endocytic cellular uptake pathway. Some distributions suggested more specific endocytic pathways. There was also a large aggregate of Au NCs freely floating in the cytosol that suggested endosomal escape. The overall analysis indicated multiple endocytic pathways were undertaken by the Au NCs. The suggested endosomal escape may be further explored as certain drugs need to be delivered directly to the cytosol. These findings agreed with the initial assertion that the pathway undertaken would be endocytic in nature.